In recent years, a photocatalytic material is attracting attention in various fields typified by the field of exterior construction as a material having a self-cleaning function, due to the oxidative decomposition effect or effect of rendering a superhydrophilic surface, caused by the irradiation of sunlight. Moreover, a photocatalytic material is also attracting attention as a useful material, in the environmental context, to potentially decompose hazardous materials including the harmful gases such as NOx in the atmosphere and the environmental hormones in water or the like. As coating compositions containing a photocatalytic material, a solvent-based coating material and a water-based coating material have been reported to date. Among them, the water-based coating material has been preferably used in various fields in view of its low impact on the environment.
A photocatalyst-containing water-based curable coating composition is composed of a photocatalytic material and a binder resin. However, when a common organic resin such as an acrylic resin and a vinyl chloride resin is used as the binder resin, organic matter decomposes due to the oxidative decomposition effect caused by the photocatalytic material. As a result, the coating films formed by the coating composition containing these organic resins were associated with the problem of reduced durability. Accordingly, as a binder resin having resistance to the oxidative decomposition effect caused by the photocatalytic material, inorganic resins such as silicon resins, and inorganic-organic composite resins composed of an inorganic component and an organic component, or the like have been proposed to date, and have been used suitably in various fields.
Photocatalyst-containing water-based coating compositions capable of forming the coating films with excellent durability as well as excellent solvent resistance, water resistance, weather resistance, stain resistance, or the like have been proposed to date. For example, a coating composition has been disclosed which is formed by a water-based medium, in which organosilane, a polymer having a silyl group containing a silicon atom bonded with a hydrolyzable group and/or a hydroxyl group, and a photocatalyst are dispersed (for example, refer to Patent Document 1).
However, in the coating composition described in the following Patent Document 1, the compatibility between polysiloxane serving as an inorganic component and an organic polymer serving as an organic component is poor. Moreover, crosslinking reaction proceeds only among the polysiloxane portion and not within the portion of polymers other than polysiloxane. As a result, it is difficult to form a coating film with excellent long term durability or weather resistance using the above coating composition.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2001-072925